In order to increase the capacity, or number of simultaneous users, of a direct-sequence CDMA (DS-CDMA) system it is desirable to enhance the detection of the signal of each user and to distinguish each signal more distinctly from noise or interference. Typically in known CDMA systems a single user detection scheme has been used in which the signal of an individual user is detected using a matched filter detector, and signals of all other users are treated by the single user detector as interference or noise. It has been proposed to enhance signal detection in a base station of a CDMA system by using a multi-user detection technique, which seeks to detect jointly the signals of multiple users.
In a known form of multi-user detection, conventional matched filtering is first used to detect a signal bit of each user, this constituting a tentative decision bit for the user. The tentative decision bits are used to regenerate the transmitted signal, including its multipath fading, for all of the multiple users. For each specific user, replica or regenerated signals for all of the other users are subtracted from a received signal mixture, thereby enhancing the specific user's signal and reducing or cancelling interference due to the other users' signals, facilitating detection of the enhanced signal for each user. Ideally, this regeneration can be repeated iteratively with successively more accurate cancellation of the respective other users' signals to achieve a correct detection of the signals of all of the users.
Inherent in this successive cancellation technique is a fundamental assumption that the initially determined tentative decision bits are error-free. Especially for large numbers of users as is a reason for providing multi-user detection, this assumption has been found not to be valid, and these first iteration tentative decision bits are not in general reliable. Consequently, the replica or regenerated signals are not reliably accurate, the subtraction of these from the received signal mixture can be wrong, and interference can be increased rather than reduced. Such error propagation has limited known multi-user detection schemes to only about 2 successive cancellation iterations, resulting in little or no gain in system capacity from the multi-user detection scheme.
A need exists, therefore, to provide improved multi-user detection in CDMA communications systems.